The present invention relates to solid state lasers, and more particularly, to chromium doped solid state lasers.
Several types of crystals have been doped with chromium for use as lasers. Alexandrite (Cr.sup.3+ :BeAl.sub.2 O.sub.4) is perhaps the most established of these. More recently, colquiriite-structure crystals (named after the tin mining region in Bolivia where the crystal structure was discovered) have been doped with chromium to produce laser materials that emit in the nearinfrared and have many desirable properties. Two such materials are Cr.sup.3+ :LiSrAlF.sub.6 (Cr:LiSAF) and Cr.sup.3+ :LiCaAlF.sub.6 (Cr:LiCAF). Both are disclosed in U.S. Pat. No. 4,811,349, issued Mar. 7, 1989 to Payne et al. When utilized as lasers, these fluorides have very low thermal lensing in comparison with oxides such as alexandrite, as well as a reduced tendency to solarize under UV light. They also provide good beam quality at sizable average power levels. Since Cr-doped lasers in general have the highest flashlamp-pump absorption efficiencies, Cr-doped fluorides such as Cr:LiCAF and Cr:LiSAF are excellent choices for tunable laser applications in the nearinfrared spectrum. They may also be frequency doubled to the blue-green region.
Although similar, the absorption and emission spectra of Cr:LiSAF are broader and shifted to longer wavelengths than Cr:LiCAF, and the absorption and emission cross-sections are larger by about a factor of four, making Cr:LiSAF the preferred crystal for many applications.
Currently, there exists a strong need for an all-solid state, tunable laser system in the near infared spectral region: as a commerical instrument replacing liquid dye lasers and titanium sapphire (TIS) lasers pumped with argon ion lasers or with frequency-doubled diode-pumped Nd-lasers; as a diagnostic source for industrial applications; as a source for isotope separation applications; as a source to be harmonically doubled into the blue-green spectral region; as a source for medical diagnostic and surgical applications. The commercially available sources mentioned above are inefficient, and, therefore, demend excessively high amounts of electrical input power and water cooling; this limits or precludes widespread use of dye and/or TIS lasers, particularly in applications requiring portability.
The "ideal" source might consist of a chromium-doped crystal laser pumped by a semiconductor laser. The principal problem to data has been determining a practical and cost-effective technical match between a powerful, and efficient semiconductor laser pump source and a chromium doped low-field crystal.
Toward this end, most recent effort in the laser R & D community has been centered on using a "red" 630-680 nm pump diode of the AlGaInP quaternary material system which is well matched to the peak of the lowest lying pump band (640 nm) of the preferred chromium crystal, Cr:LiSAF. The use of a mature, efficient, and powerful pump diode from the AlGaAs ternary material system, which emits in the 740-880 nm spectral region, has been completely ignored by the laser community, because of the perception that such a diode cannot be used to effectively pump Cr:LiSAF.